Prosecutors on Thursday grilled three former and incumbent senior executives of SK Group in connection with an ongoing investigation into ousted President Park Geun-hye over bribery charges.

The nation's third-largest business group donated a total of 11.1 billion won ($9.82 million) to two dubious foundations allegedly controlled by Park's friend Choi Soon-sil between 2015 and 2016.

Prosecutors are looking into whether the donations were in return for the government's pardon in 2015 of group chief Chey Tae-won, who was serving time for embezzlement and other financial crimes. They are also investigating whether the group was seeking Choi's influence with the government to help it win licenses for lucrative duty-free business.

Kim Chang-geun, who headed the SUPEX Council, the group's top decision-making body, from 2012 to 2016, appeared at the Seoul Central District Prosecutors' Office in southern Seoul to undergo questioning. The council is comprised of the chiefs of each affiliate.

Kim Young-tae, who heads the council's communication committee and Lee Hyung-hee, chief of the group's TV service unit SK Broadband, were also questioned on the same day.

The interrogations came as prosecutors plan to question the country's former president next Tuesday over multiple criminal allegations, including that she colluded with Choi in extorting large sums of money from local conglomerates.